Web metering mechanism

ABSTRACT

A metering mechanism utilized for form-out of a longitudinally movable succession of serially attached identical forms, is adapted to change the mode of form advancing means in a printing apparatus selectively as a function of form length to facilitate initiation of a printing operation at a common level on each form. The mechanism is adapted to accommodate forms of varying lengths by a preset adjustment of the oscillating angle between a pair of yieldably coupled angularly adjustable rotatable discs which selectively shift a rocking member having a pair of driving gears thereon between two stable angular positions to permit the gears alternately to engage alternate ones of the angularly adjustable discs. The mode of the advancing means is changed after each shift of the rocking member.

United stat s Patent [191 H Hagstrom 1 Sept. 25, 1973 i l WEB METERING MECHANISM Primary ExaminerAllen N. Knowles [75] Inventor: Arthur A. Hagstrom, Hoffman Asfmmm Emmmer,Gene Church Esates "L Attorney-J. L. Landls et al.

[73] Assignee: Teletype Corporation, Skokie, Ill. [22] Filed: Aug. 31, 1972 [57] ABSTRACT [21 1 AppL No: 285,286 A metering mechanism utilized for form-out of a longitudinally movable succession of serially attached identical forms, is. adapted to change the mode of form ad- [52] US. Cl. 226/37, 226/76, 226/1'52, in means in a printing apparatus selectively as a 226/ 1 83, 197/133 R, 226/134 function of form length to facilitate initiation of a print- [51] Int. CI B65II 23/18 ing operation at a cgmmon level on each fen- The of Search 76, mechanism is adapted to accommodate forms of vary- 226/l8 197/133 133 P ing lengths by a preset adjustment of the oscillating angle between a pair of yieldably coupled angularly adl l References Cited justable rotatable discs which selectively shift a rocking UNITED STATES PATENTS member having a pair of driving gears thereon between 3,107,544 10/1963 Nichols et all 226/134 two Stable angular Positicms Permit the gears alter- 3,125,309 3/1964 Mayand 226/134 x nately to engage alternate ones of the angularly j 3,l70,6l4 2/1965 Faust 226/ 134 able discs. The mode of the advancing means is 3,394,853 7/1968 Folet et al... 226/37 X changed after each shift of the rocking member. 3,460,343 8/1969 Toushman 20 Claims, 7 Drawing Figures 1 WEB METERING MECHANISM BACKGROUND OF THE INVENTION The present invention relates to a web metering mechanism, and more particularly to a form-out mechanism.

In automatic form printing apparatuses, for example, I

The operation of moving successively presented 1 forms through a printing apparatus longitudinally of a web path for a distance equal to the length of each form to initiate printing at a common level thereon is known as form-out." Previously, form-out has been carried out by mechanical as well as electronic means, for example, counters which control the advancement of the stepping motor. Such control is relatively expensive and predisposed to errors such that printing may not be initiated at a common level on each form.

SUMMARY OF THE INVENTION It is an object of the present invention to control a web advancing means selectively to advance a web a predetermined longitudinal distance.

A further object of the present invention is to avoid the errors inherent in electronic web metering systems and provide improved reliability in form-out.

It is' a further object of the present invention to adjust a form-out mechanism to accommodate forms of different lengths.

With these and other objects in view, a web metering mechanism for selectively changing the mode of operabetween first and second stable positions, first and sec- I ond driving elements rotatably supported on the rockin g member, the first driving element drivably engaging the first rotatable member when the rocking member is in the first stable position and the second driving element drivably engaging the second rotatable element when the rocking member is in the second stable position, means responsive to the operation of the web advancing means for rotating the driving elements simultaneously in opposite directions to rotate the rotatable members in the direction of the driven one thereof,

shifting means in fixed position on a segment of each of the rotatable members for selectively urging its associated driving element out of engagement therewith to shift the rocking member from one stable position to the other stable position to urge the driving element associated with the other rotatable member into driving engagement therewith alternately to reverse the direction of rotation of the rotatable members and means responsive to the shifting of the rocking member to interrupt the operation of the web advancing means.

2 BRIEF DESCRIPTION OF THE DRAWINGS The objects and various features of the invention will be more readily understood from the following detailed description of a specific embodiment thereof, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view of a web metering mechanism in accordance with the principles of the present invention;

FIG. 2 is similar to FIG. 1, though alternately conditional, with a portion of the mechanism broken away to show the relationship of a set of selectively engageable driving gears carried by a rocking element;

FIG. 3 is a sectional view of the web metering mechanism of FIG. 1, taken along the lines 33 thereof;

FIG. 4 is a sectional view of the web metering mechanism of FIG. 1, taken along the lines 44 thereof;

FIG. 5 is a partial sectional view of a ball detent coupling device of the web metering mechanism taken along the lines 5--5 of FIG. 4;

FIG; 6 is similar to FIG. 4, with a portion of the apparatus broken away to show a second ball detent mechanism utilized for selectively adjusting the angular relationship between a pair of driven discs; and

FIG. 7 is similar to FIG. 1, with a portion of the apparatus broken away to show a locking means for the web metering mechanism.

DETAILED DESCRIPTION Referring to FIG. 1, a web metering mechanism 20 is utilized to control the advancement of a plurality of serially-connected forms, such as the form 21 shown in FIG. 4, from a fan-folded supply thereof (not shown) through a printing apparatus, for example, a line-at-atime teleprinter. Each form 21 has, at the edges thereof, equally spaced sprocket holes 22-22 to facilitate form advancement, for example, in the direction shown by arrow 24. A plurality of aligned circumferentially spaced sprocket pins 26-26 carried at each end of a cylindrical platen 23 (only one end of which is shown in FIG. 4) are proportioned for reception in holes 2222 for advancing the forms 21 as the platen 23 is rotated. The sprocket pins 26-26, the platen 23, and an adjustable web driving circuit 25 comprise web advancing means. Circuit 25 may be any heretofore known construction and includes a stepping motor (not shown) of a known type; and. said circuit has several modes in which the time intervals between web advancing steps are different.

The platen 23 is secured to a rotatable shaft 27 journalled in a bearing '28 supported by the frame of the printing apparatus, an end plate 31 of which is shown in FIGS. 1, 4, 5, and 7. The shaft 27 is coupled to a driving gear 32 formed with a plurality of teeth 33-33 which mesh with a plurality of teeth 34-34 formed around the periphery of a clutch member 35 journalled about a flanged sleeve 36 disposed on a shaft 37 fixedly secured to the end plate 31. A re-.

duced cylindrical portion of the clutch member 35 is formed with a plurality of radially arranged grooves 38-38, as shown in FIG. 5. These may have V-shaped cross-sections, which define an annular 43 for receiving a retaining member 44 including an annulus 46 (FIG. having three equally spaced resilient arms 47-47 extending radially therefrom. As shown in FIGS. 4 and 5, each arm 47 is formed with an opening 48 for receiving a ball detent 51 between the retaining member 44 and a notch 38 on the face of the clutch member 35.

A toothedwheel 52 keyed to the hub 42 of gear 41,

is formed with a plurality of circumferentially spaced teeth 53-53. The wheel 52 is provided with at least three detent pockets or depressions 56-56, which are alignable with the arms 47-47 for receiving the ball 4 to urge the ball detents 51-51 into selected ones of the grooves 38-38 on the clutch member 35 to provide a drive coupling between the wheel 52 and the clutch member 35. An annular disk 62, formed with a plurality of circumferentially spaced apertures 63-63, as illustrated in FIGS. 1, 2, 3, and 7, is secured to-the wheel 52 on a hub 64 formed thereon.

As best seen in FIGS. 1, 3, and 4, a rocking member or hanger 65 is rigidly secured to the bushing element 58 and thereby is adapted to rock about the shaft 37. The hanger 65 supports a first transfer gear 66, an idler 67 and a second transfer gear 68 on three parallel dowels 69, 70, and 71, respectively, fixed to the hanger 65.

The gear 66 and the idler 67 are indriving engage ment with the gear 41. The idler 67 also operably meshes with the gear 68 such that the transfer gears 66 and 68 are driven in opposite directions by the rotation of gear 41.

As shown in FIGS. 1 and 2, the hanger 65 is forme with an extended portion 72 having a head 73 formed with first and second angularly related camming surfaces 76 and 77, each of which is selectively engageable by a follower 78 supported at one end of an arm 81 pivotally supported at its opposite end on a pin 82. An expansion spring 83 has one end connected to the arm 81 and its other end connected to pin 84. Both pins 82 and 84 project normally from a support plate 85 spaced from and in planar parallel relationship with the end plate 31, the arrangement of parts adapted to urge the follower 78 against one of the camming surfaces 76 or 77 on the head 73, resiliently to retain the hanger 65 in one of two angular positions.

In the first angiilar position of the hanger 65 (FIG. 1) the follower 78 is urged into engagement with the carnming surface 76 on the head 73, with the gear 68 in driving engagement with a first rotatable toothed disc 86, to drive disc 86 clockwise.

In the second angular position of the hanger FIG. N

2), the follower 78 is urged into engagement with the 1972 in the name of Hans Keipert and assigned to the assignee of the present invention. Reference to the foregoing disclosure may be made for a more detailed description of the operation thereof.

The discs 86 and 87 have a plurality of peripheral, circumferentially spaced teeth 91-91 and 92-92, respectively. Additionally, the disc 86 is provided with a projection between a pair of adjacent teeth 91-91 to form a lug 93 (FIG. 1) therewith, which may have a circumferential width equal to two or more of the teeth 91-91 and which may project radially beyond the circumference formed by the outer edges of teeth 91-91. The lug 93 may be a removable radial projection or one welded on the disc 86 and need not be formed on the disc. The lug 93 breaks the driving relationship between the transfer gear 68 and angle which may be selectively varied by adjusting the angular relationship of the discs about their common axis. Utilizing the nomenclature set forth in the abovereferenced application of Keipert, the disc 86 will be referred to as the clutch disc and the disc 87 will be referred to as the biased" disc.

The biased disc 87 is disposed on a hub 95 formed on the clutch disc 86 and is urged toward the disc 86 by a compression spring 98 on the hub 95.

The inner surface of the clutch disc 86, facing the biased disc 87, is formed with a plurality of radial V- shaped grooves 101-101 similar to the grooves 38-38 formed in the clutch member 35. The inner surface of the biased disc 87 is formed with three detent pockets or depressions 102-102, similar to the pockets 56-56 formed in the wheel 52, equally spaced circumferentially therearound and alignable with the grooves 101-101.

Referring to FIGS. 1, 4, and 6, three ball detents 103-103 are disposed in the detent pockets 102-102 and are retained therein by a resilient retainer 104 (FIG. 6) formed with three resilient arms 105-105, y one of t g z s l w i95 er 19.5 has. diiidiigiii slot 106 formed at the outermost end thereof. The retainer 104 is disposed in a circumferential groove (not shown) formed in the hub 95, similar to the circumferential groove 43 for the retainer 44. The curved surface of each ball detent 103 protrudes through the slot 106 in each retainer arm 105, into latching engagement with a selected oneof the plurality of V-shaped grooves 101-101 formed in the clutch disc 86. This provides a yielding or detentable or drive coupling between the discs 86 and 87.

Additionally, the discs 86 and 87 are provided with laterally extending pins 111 and 112 (FIG. 1), respectively, between the facing surfaces of the discs. Pin 1 1 1 is in a fixed position on the disc 87 in a position spaced a predetermined angular distance behind the lug 93 thereon as the disc 86 is rotated in its driven clockwise direction by the transfer gear 68. Similarly, pin 112 is spaced a predetermined angular distance behind the lug 94 on the disc 87 as the latter is rotated in its driven counterclockwise direction by the transfer gear 66.

The angular relationship of the discs 86 and 87 can be selectively adjusted to any desired one of a plurality of possible angular relationships by relative manual rotation of the discs 86 and 87. The ball detent coupling mechanism described will permit one disc to be rotated incrementally with respect to the other. One disc may remain fixed while the other is rotated to permit selective angular orientation therebetween. The angular relationship of the discs 86 and 87 is determinative of the angular movement or oscillation thereof about. the shaft 88, thus the angular distance through which the discs 86 and 87 are rotated before reversal by the hanger 65, may be adjusted by the relative angular adjustment of the discs 86 and 87. This angle can be directly related to the length of the form 21 which is to undergo a printing operation.

An arm 1 16 is secured on the bushing element 59 for pivotal movement around the shaft 37. An expansion spring 1 17 urges the arm 1 16 to a normal light-blocking condition (FIG. 1). For that purpose, one end of the spring 117 is anchored on arm 116 and the other end of spring 1 17 is anchored on a post 119 (FIG. 1) which is secured to the plate 85. In its light-blocking position, the arm 116 is interposed between a light source 118 (FIG. 3), for example, a lamp, supported in a fixed position, and a photo sensor or photo receptor 121 which is supported from plate 85, as shown in FIG. 3 and opare driven angularly, either the pin 111 or the pin 112, 7

depending upon the angular direction of movement, will engage and pivot the arm 116 against the bias of spring 1 17 to a light unblocking or light passing condition, as shown in FIG. 2 to permit the light from source 118 to impinge upon the photo sensor 121 through one of the apertures 63 then disposed in alignment with the source 118 and photo sensor 121.

Additionally, a pawl 122, as shown in FIG. 7, is mounted pivotally on a pin 123 secured to the plate 85. A slide 124 is loosely mounted on the plate 31 by a pair of pins 126 and 127 extending through a pair of elongated slots I28 and 131 formed in the slide 124. A drive pin 125 carried by the slide projects through one end of the pawl 122 for rocking its latching head 129 between the dotted and solid line positions of FIG. 7. An expansion spring 130 urges the pawl 122 into locking solid line position between a pair of adjacent teeth 53-53 on the wheel 52. One end of this spring 130 is connected to the pawl 122. The other end of said spring 130 is anchored on a post 135 (FIGS. 1, 3, and 7) rigidly secured to plate 31. The pawl 122 is adapted releasably to lock the wheel 52 against movement to permit the initial loading of a form 21 in the printing apparatus by a manual rotation of the platen 23 which causes gears 32 and 35 to rotate with the teeth 38-38 on the clutch member 35 being racheted past the ball detents 51-51, transmitting no motion to the remainder of the web advancing mechanism 20.

The pawl 122 may be operated by operatively connecting the slide 124 to a hinged cover or the like (not shown) on the printing apparatus which may be opened to provide access to the web metering mechanism 20 and which when closed moves the slide 124 downwardly to rock the head 129 to its dotted line position for releasing wheel 52.

To set the arcuate spacing of pins 111 and 112 according to the length of the form 21, an arcuate scale of indicia may be secured to disc 87 coaxially therewith, preferably near the periphery of said disc 87. The spacing of indicia on scale 132 corresponds to the line of spacing on form 21. A fixed pointer 133 is secured to the frame of mechanism 20 such that upon alignment of the pointer 133 with a fixed point 134 (FIG. 1) on disc 86, a mark of indicia on scale 132 indicates the period of mechanism 20, that is, the arcuate distance through which discs 86 and 87 will travel before reversal of direction in accordance with the present invention.

OPERATION OF THE ILLUSTRATED EMBODIMENT During printing, the motor comprising the adjustable web drive circuit 25 is operated in a first mode in which a form 21 is advanced a line each time a line of printing has been completed. The logic for generating this effect may be conventional. Initiation of this first mode is responsive to a signal generated upon exposure of the photo sensor 121 to the light source 118 in consetiu e rice oT the 215E116 being moved to its light passing condition (FIG. 2). Upon a signal generated by a known means (not shown) upon completion of a printed message on each form, the mode of operation of the adjustable web drive circuit 25 is changed for rapidly advancing the succession of forms such that the level of the first line intended for print on an uprinted form following a printed form is aligned with the printing means (not shown). As this last position is reached the web advancing mechanism 20 is completing a cycle and arm 116 is being moved to a light passing condition, for the next cycle.

During the period in which the web moves between a first line on a form 21 to a first line on an ensuing form, the discs 86 and 87 will travel in one angular direction. Thereupon, the direction of the discs 86 and 87 will reverse and will move in an opposite angular direction for an ensuing period of time.

To initially set the mechanism 20 for a succession of forms 21, in accordance with the invention the discs 86 and 87 should be rotated in the direction shown by arrow 136 (FIG. 2) until the lug 94 has engaged and shifted hanger 65 just beyond the condition shown in FIG. 2 to the condition of FIG. 1. At this time the coupled discs will now be conditioned for moving in the direction of arrow 138 (FIG. 1) and arm 116 will be in a light passing condition as shown in FIG. 2. Simultaneously'drive circuitry 25 will be in its first-or printing mode for advancing a form only when a line of printing has been completed.

At this time disc 86 should be immobilized and pointer 133 is aligned with point 134. Then, disc 87 can be manually moved relative to disc 86 to set the rocking period of the mechanism 20 according to the length of each of the forms 21 by aligning a proper index mark on scale 132 with the pointer 133.

In accordance with the present invention, the discs 86 and 87 can be recoupled by releasing disc 87; and a succession of forms 21 may now be inserted in the mechanism 20 for aligning its fust line for print with the printing mechanism (not shown). The adjustment of the form can be achieved without moving discs 86 and 87 (set as aforesaid) by rocking the platen 23 with the sprocket pins 26-26 drivingly engaged in the holes 22-22, while the slide 124 is elevated. Thereby pawl 122 will be in its latched condition (solid line of FIG. 7) for immobilizing wheel 52. Consequently, torque of the platen 23 will not be transmitted to the gear 41.

Thereafter, upon elevation of slider 124, form-out can en'sue automatically, as the pawl 122 will disengage the wheel 52. Accordingly, as the platen 23 is driven by the web driving circuitry 25, its torque will be transmitted through ball detents 51 to the gear 41 which will move responsively about shaft 37. This movement will now cause the coupled discs 86 and 87 to move clockwise, in the direction of arrow 138 (FIG. 1). Upon initiation of such last movement, pin 112 will move away from arm 1 16, which, under the bias of spring 117, will move to its normal lightblocking position. Movement will continue thereafter, responsively to movement of the platen 23, until the pin 111 engages arm 116 and drives it angularly from its position of FIG. 1 counterclockwise against the bias of spring 117 and hanger 65 is shifted by lug 93 to its driving condition of FIG. 2. Thereupon, a reverse period of the web mechanism 20 will ensue.

Attention is invited to the fact that the mode of the web advancing means in the exemplary web advancing means changes at the end of a printed message (which may not necessarily be at the end of a form). At that time in response to a signal generated by means (which may be conventional), the web drive circuit 25 changes from a printing mode to a non-printing mode and remains in said last mode until the web advancing mechanism 20 completes one of its periods. The angular oscillation of coupled discs 86 and 87 between pins 111 and 112 continues as long as desired, in consequence of the foregoing construction, the direction of movement being responsive to the alternations of hanger65 which are effected by the moving discs 86 and 87 themselves. While' the preferred embodiment of the web metering mechanism 20 has been described in connection with a printing apparatus, it will be understood that it may be used with various types of marking or length measuring apparatuses for various types of elongated material. lt is to be understood that various other embodiments and uses of the invention may be devised by those skilled in the art, which will employ the principles of the invention and fall within the spirit and scope thereof.

I claim: I 1. A web metering mechanism for selectively changing the operational mode of a web advancing means, which comprises:

first and second concentrically aligned rotatable members; means releasably coupling said rotatable members for simultaneous movement; a rocking member shiftable between a first stable position and a second stable position; first and second driving elements rotatably supported on said rocking member, said first driving element being in driving engagement s a il first rotatablemember when said rocking member is in said first stable position and said second driving element being in driving engagement with said second rotatable member when said rocking member is in said second stable position;

means for rotating said driving elements simultaneously in opposite directions to rotate said rotat: are members simultaneously in the direction of the driven one thereof;

means in fixed position on a segment of each of said rotatable members for shifting the driving element associated therewith out of driving engagement therewith to shift said rocking member from one stable position to the other stable position to urge the driving element associated with the other rotatable member into driving engagement therewith alternately to reverse the direction of rotation of said rotatable members; and

means responsive to the rotatable members for changing the mode of said web advancing means periodically.

2. A web metering mechanism as set forth in claim 1 wherein said driving elements are toothed gears said rotatable members are angularly adjustable toothed discs and said shifting means is a radial projection on each of said toothed discs.

3. A web metering mechanism as set forth in claim 2 wherein said rocking member is pivotable between two positions with respect to an axis therethrough, each of said toothed discs includes a lateral projection parallel to said axis at a predetermined relationship to said radial projection, and said interrupting means includes an arm mounted for pivotal movement with respect to said axis and engageable by the lateral projection on each disc to pivot said arm during a portion of the time each disc is driven.

4. A web metering mechanism as set forth in claim 3 wherein said interrupting means further includes a light source in axial alignment with a photoreceptor, and said arm is normally interposed in the axial light path between said light source and said photoreceptor.

5. A web metering mechanism as set forth in claim 4 wherein the lateral projection on the driven one of said discs engages and pivots said arm to unblock said light path.

6. A web metering mechanism as set forth in claim 5 wherein said lateral projection on said driven disc engages and pivots said arm when said radial projection on said driven disc shifts said rocking member.

7. A web metering mechanism as set forth in claim 6 wherein the rotational angle through which the discs are driven is a function of the angular distance between said radial projections.

8. A web metering mechanism as set forth in claim 7 wherein the angular distance between said radial projections is directly related to the length of web material advanced by the web advancing means between reversals of said discs. I

9. A web metering mechanism as set forth in claim 8 wherein said releasable coupling means includes a resilient means for permitting the relative angular adjustment of said discs to adjust the angular distance between said radial projections.

10. In a printing apparatus including a rotatable platen, a motor driving said platen for advancing a plurality of serially attached identical forms longitudinally past a printing means for a. distance equal to the length of each form to initiate a printing operation at a common level on each form;

first and second concentrically aligned toothed discs, each including a radial projection on the toothed surface thereof, the angulardistance between the radial projections being directly related to the length of each form;

means for releasably coupling said discs for simultaneous movement;

a rocking member shiftable from a first stable angular position about an axis to a second angular position thereabout;

first and second gears supported on said rocking member, said first gear drivably engaging said first disc when said rocking member is in first position and said second gear drivably engaging said second disc when said rocking member is in said second position;

means coupled to said platen for transmitting the rotary movement thereof to said gears to drive said gears simultaneously in opposite directions to rotate said discs simultaneously in the direction of the driven one thereof until the radial projection on the driven disc engaged the driving gear associated therewith to urge said gear out of engagement with said driven disc, shifting said rocking member from one stable position to the other stable position to urge the oppositely rotating gear associated with the other disc into driving engagement therewith to reverse the direction of rotation of said discs;

a light source;

a photoreceptor axially aligned with said light source;

an arm mounted for pivotal movement about the axis of said rocking member and having an end thereof projecting between said discs, the other end thereof being normally interposed in the axial path between said light source and said photoreceptor;

a pin fixed to each disc at a predetermined radial distance from the axis thereof and at a predetermined angular distance from the radial projection thereon, for engaging and pivoting said arm when said radial projection shifts said rocking member to permit the light from said source to activate said photoreceptor; and 1 means responsive to the activation of said photoreceptor for stopping the motor driving said platen.

11. In a printing apparatus as set forth in claim and including means for isolating said gears from said platen to permit an initial adjustment of said platen without rotating said discs.

12. In a printing apparatus as set forth in claim 10 wherein said releasable coupling means includes resilient means for permitting relative angular adjustment of said discs according to form length.

13. In a web metering mechanism of the type in which a pair of releasably coupled coaxially disposed rockable members alternately are driven in each of opposed angular directions and the angular distance of movement of said members corresponds to the distance of web movement, the improvement in means for changing the angular direction of movement of said members comprising: 7

an assembly including a pair of member driving elements, each element mounted for rotation in an angular direction opposite to the direction of rotation of the other element;

means for driving said elements according to the distance of web movement; and

{said hanger has a first condition in which said first ldriven gear is drivingly engaged for rotating said rotatable members in one angular direction and a second lcondition in which said second driven gear is drivingly engaged for rotating said rotatable member in another Iangular direction.

means carried on said rotatable members for shifting said assemblyalternately to move first one of said driving elements into driving engagement with one of said members and then the other of said driving elements-into driving engagement with the other of said members.

14. A combination according to claim 13 wherein said driving means comprises a drive gear and said driving elements comprise a pair of driven gears coupled to said drive gear, said combination further characterized by a shiftable hanger included in said assembly, said driven gears rotatably mounted on said hanger.

15. A combination according to claim 14 wherein said drive gear has a fixed axis of rotation, said drive rotatable, when operating, only in single angular direction, wherein said hanger is rockably mounted about said axis for shifting, and wherein each of said driven gears has a discrete axes of rotation and, when operating, rotates in an angular direction opposite to the direction of rotation of the other driven gear.

16. A combination according to claim 15 wherein 17. A combination according to claim 16 characterized by teeth disposed along an arcuate line on each rotatable member for meshing with said driven gears and a pair of angularly spaced apart lugs disposed along said lines for engaging said driven gears and alternately shifting said hanger to its first and second conditions.

18. A web metering mechanism comprising:

means for advancing a web equal successive incremental distances and having a pair of modes during which the time interval between incremental web movements are different;

a pair of members disposed about an axis and rotating an angular distance corresponding to the distance of web advance;

means for converting unidirectional torque accompanying web advance to a pair of torques of opposed directions and periodically driving said members alternately first in one angular direction and then in another angular direction about said axis;

a pair of angularly spaced apart projections carried on said members;

means for releasably securing each member in a selected adjusted angular position about said axis relative to the other member to adjust the spacing of said projections; and

a movable element disposed in the path of movement of said projections for changing the mode of said web advancing means.

19. A combination according to claim 18 further characterized by:

an apertured disc mounted for angular movement in accordance with the distance of web movement;

sensing means having parts arranged in alignment on opposite sides of said disc for energy communication through said apertures;

a shade as an extension of said movable element having a first condition for blocking energy passage between said parts and an energy passing condition.

20. A combination according to claim 19 further characterized by means for yieldably urging said shade to said first condition and tensioned upon movenent of said shade to said second condition at the end of each period of alternation of said members. 

1. A web metering mechanism for selectively changing the operational mode of a web advancing means, which comprises: first and second concentrically aligned rotatable members; means releasably coupling said rotatable members for simultaneous movement; a rocking member shiftable between a first stable position and a second stable position; first and second driving elements rotatably supported on said rocking member, said first driving element being in driving engagement with said first rotatable member when said rocking member is in said first stable position and said second driving element being in driving engagement with said second rotatable member when said rocking member is in said second stable position; means for rotating said driving elements simultaneously in opposite directions to rotate said rotatable member simultaneously in the direction of the driven one thereof; means in fixed position on a segment of each of said rotatable members for shifting the driving element associated therewith out of driving engagement therewith to shift said rocking member from one stable position to the other stable position to urge the driving element associated with the other rotatable member into driving engagement therewith alternately to reverse the direction of rotation of said rotatable members; and means responsive to the rotatable members for changing the mode of said web advancing means periodically.
 2. A web metering mechanism as set forth in claim 1 wherein said driving elements are toothed gears, said rotatable members are angularly adjustable toothed discs and said shifting means is a radial projection on each of said toothed discs.
 3. A web metering mechanism as set forth in claim 2 wherein said rocking member is pivotable between two positions with respect to an axis therethrough, each of said toothed discs includes a lateral projection parallel to said axis at a predetermined relationship to said radial projection, and said interrupting means includes an arm mounted for pivotal movement with respect to said axis and engageable by the lateral projection on each disc to pivot said arm during a portion of the time each disc is driven.
 4. A web metering mechanism as set forth in claim 3 wherein said interrupting means further includes a light source in axial alignment with a photoreceptor, and said arm is normally interposed in the axial light path between said light source and said photoreceptor.
 5. A web metering mechanism as set forth in claim 4 wherein the lateral projection on the driven one of said discs engages and pivots said arm to unblock said light path.
 6. A web metering mechanism as set forth in claim 5 wherein said lateral projection on said driven disc engages and pivots said arm when said radial projection on said driven disc shifts said rocking member.
 7. A web metering mechanism as set forth in claim 6 wherein the rotational angle through which the discs are driven is a function of the angular distance between said radial projections.
 8. A web metering mechanism as set forth in claim 7 wherein the angular distance between said radial projections is directly related to the length of web material advanced by the web advancing means between reversals of said discs.
 9. A web metering mechanism as set forth in claim 8 wherein said releasable coupling means includes a resilient means for permitting the relative angular adjustment of said discs to adjust the angular distance between said radial projections.
 10. In a printing apparatus including a rotatable platen, a motor driving said platen for advancing a plurality of serially attached identical forms longitudinally past a printing means for a distance equal to the length of each form to initiate a printing operation at a common level on each form; first and second concentrically aligned toothed discs, each including a radial projection on the toothed surface thereof, the angular distance between the radial projections being directly related to the length of each form; means for releasably coupling said discs for simultaneous movement; a rocking member shiftable from a first stable angular position about an axis to a second angular position thereabout; first and second gears supported on said rocking member, said first gear drivably engaging said first disc when said rocking member is in said first position and said second gear drivably engaging said second disc when said rocking member is in said second position; means coupled to said platen for transmitting the rotary movement thereof to said gears to drive said gears simultaneously in opposite directions to rotate said discs simultaneously in the direction of the driven one thereof until the radial projection on the driven disc engaged the driving gear associated therewith to urge said gear out of engagement with said driven disc, shifting said rocking member from one stable position to the other stable position to urge the oppositely rotating gear associated with the other disc into driving engagement therewith to reverse the direction of rotation of said discs; a light source; a photoreceptor axially aligned with said light source; an arm mounted for pivotal movement about the axis of said rocking member and having an end thereof projecting between said discs, the other end thereof being normally interposed in the axial path between said light source and said photoreceptor; a pin fixed to each disc at a predetermined radial distance from the axis thereof and at a predetermined angular distance from the radial projection thereon, for engaging and pivoting said arm when said radial projection shifts said rocking member to permit the light from said source to activate said photoreceptor; and means responsive to the activation of said photoreceptor for stopping the motor driving said platen.
 11. In a printing apparatus as set forth in claim 10 and including means for isolating said gears from said platen to permit an initial adjustment of said platen without rotating said discs.
 12. In a printing apparatus as set forth in claim 10 wherein said releasable coupling means includes resilient means for permitting relative angular adjustment of said discs according to form length.
 13. In a web metering mechanism of the type in which a pair of releasably coupled coaxially disposed rockable members alternately are driven in each of opposed angular directions and the angular distance of movement of said members corresponds to the distance of web movement, the improvement in means for changing the angular direction of movement of said members comprising: an assembly including a pair of member driving elements, each element mounted for rotation in an angular direction opposite to the direction of rotation of the other element; means for driving said elements according to the distance of web movement; and means carried on said rotatable members for shifting said assembly alternately to move first one of said driving elements into driving engagement with one of said members and then the other of said driving elements into driving engagement with the other of said members.
 14. A combination according to claim 13 wherein said driving means comprises a drive gear and said driving elements comprise a pair of driven gears coupled to said drive gear, said combination further characterized by a shiftable hanger included in said assembly, said driven gears rotatably mounted on said hanger.
 15. A combination according to claim 14 wherein said drive gear has a fixed axis of rotation, said drive rotatabLe, when operating, only in single angular direction, wherein said hanger is rockably mounted about said axis for shifting, and wherein each of said driven gears has a discrete axes of rotation and, when operating, rotates in an angular direction opposite to the direction of rotation of the other driven gear.
 16. A combination according to claim 15 wherein said hanger has a first condition in which said first driven gear is drivingly engaged for rotating said rotatable members in one angular direction and a second condition in which said second driven gear is drivingly engaged for rotating said rotatable member in another angular direction.
 17. A combination according to claim 16 characterized by teeth disposed along an arcuate line on each rotatable member for meshing with said driven gears and a pair of angularly spaced apart lugs disposed along said lines for engaging said driven gears and alternately shifting said hanger to its first and second conditions.
 18. A web metering mechanism comprising: means for advancing a web equal successive incremental distances and having a pair of modes during which the time interval between incremental web movements are different; a pair of members disposed about an axis and rotating an angular distance corresponding to the distance of web advance; means for converting unidirectional torque accompanying web advance to a pair of torques of opposed directions and periodically driving said members alternately first in one angular direction and then in another angular direction about said axis; a pair of angularly spaced apart projections carried on said members; means for releasably securing each member in a selected adjusted angular position about said axis relative to the other member to adjust the spacing of said projections; and a movable element disposed in the path of movement of said projections for changing the mode of said web advancing means.
 19. A combination according to claim 18 further characterized by: an apertured disc mounted for angular movement in accordance with the distance of web movement; sensing means having parts arranged in alignment on opposite sides of said disc for energy communication through said apertures; a shade as an extension of said movable element having a first condition for blocking energy passage between said parts and an energy passing condition.
 20. A combination according to claim 19 further characterized by means for yieldably urging said shade to said first condition and tensioned upon movement of said shade to said second condition at the end of each period of alternation of said members. 